for all \(a,b\in B\) . By [1]}, we have that \(ab\in G\) , for all \(a,b\in B\) . Therefore, if \(X\)  and \(Y\)  are subsets of \(B\) , we denote by \(XY\)  the additive subgroup of \((G,+)\)  generated by the elements of the form \(ab\) , where \(a\in X\)  and \(b\in Y\) . In the case \(X=Y:=B\)  one can give two further notions which will be important in this section.
